The simple eukaryote Saccharomyces cerevisiae provides a good genetic system and a source of highly purified DNA replication and repair proteins for the study of molecular mechanisms of mutagenesis. A major effort of the past two years has been to determine the fidelity of DNA synthesis by the major replicative polymerase in yeast, DNA polymerase I. To examine the possibility that subunits other than the polymerase catalytic subunit may influence fidelity, we measured the accuracy of yPolI purified by conventional procedures, which yields polymerase with a 140 Kd catalytic subunit and no associated primase activity, and that of yPolI purified by immunoaffinity chormatography, which yields polymerase having a catalytic subunit of approximately 180 kd as well as three additional polypeptides and primase activity. The average fidelity was similar for both forms of polymerase and demonstrate that, like its equivalent in higher eukaryotes (polymerase alpha), both yPolI and the polymerase I-primase complex produce three predominant classes of errors, single-base substitutions, single-base frameshifts and larger deletions. For specific errors and template positions, the two forms of polymerase do exhibit interesting fidelity differences. Despite these differences, when considering the overall error frequency and the spectrum of DNA synthesis errors, the results suggest that the polymerase I-primase complex is not highly accurate, and, just as for the polymerase alone, its fidelity is not sufficient to account for low spontaneous mutation rates in vivo. The mutant specificity data also suggest models to explain two subsets of errors. First, we propose that in the appropriate sequence context certain base misinsertion events can rearrange to form stable misalignments that lead to -1 frameshifts even in nonreiterated sequences. Results of DNA polymerase reactions with specifically designed mismatched template-primers support this model. Second, among the deletions, many of which are between direct repeats, is a subset that contain unusual junctions. These can be explained by aberrant "loop-back" synthesis by yPolI to generate complex deletions by a direct repeat mechanism.